1. Field of the Invention
The present invention relates to a method of calibrating a thermal trigger for an electrical switching device. The method is especially useful for calibrating a thermal trigger having one bimetal strip for each phase, wherein one end of each bimetal strip is attached to the housing of the trigger, the thermal trigger also having two longitudinally displaceable positioned slides arranged in parallel, wherein the free end sections of the bimetal strips that are not attached to the inside of the housing and which bend outward under current loading in the direction of longitudinal displacement of the slides lie between both of the slides, the thermal trigger also having a differential lever which is linked to one slide via a link joint and to the other slide via a tow connection, and which acts with at least one end section upon a trigger arrangement, and the thermal trigger also having drive arms between each slide and each bimetal strip.
2. Description of Related Art
ED-A1 0302822 describes a method of the above-mentioned type. This calibration method is designed for thermal triggers with a bimetal strip for each phase with two parallel arranged longitudinally displaceable positioned slides, wherein the end sections of the bimetal strips, that under current loading bend outward in a direction of longitudinal displacement of the slides, lie between the slides. A differential lever actuating a trigger arrangement with at least one end section is attached to one slide by way of a link-joint, and to the other by way of a tow connection. A sliding, relocatable slide unit is surface-mounted onto the end section of each bimetal strip enclosing it and leaving some play, and which slides on the slides in the longitudinal direction of the slides before calibration. Thereafter, the bimetal strips are heated with the appropriate calibration current using the lowest tripping current according to regulations, until the slide units that are pushed along the slides by the outward bending bimetal strips have attained the trigger point of the tripping arrangement. The slide units are attached on both slides in this position, preferably by ultrasonic welding.
The major disadvantages of this method are that this method is unsuitable for automatic assembly and that this method permits unacceptably large tolerances (because the openings of the slide units that are clinched onto the end sections of the bimetal strips enclose the bimetal strips leaving play, and the openings also do not allow for variance in thickness among the bimetal strips). In addition, undefined displacements due to slip between the slide units and the slides can occur during ultrasonic welding. Furthermore, the slide units have to be cut open after calibration, resulting in additional costs for the proper disposal of waste.